I know he sacrificed himself but what did he do
They talked about it a lot earlier in the story (remember when they went to see the guy they thought was the teacher's boyfriend) about how Shirone can do more intense and complicated magic but would basically give up his sense of self and control of himself, basically the path of no return. I think it's similar to the concept of transcendence, or Nirvana? Understanding and becoming one with something but giving up your sense of self/your existence as a whole, I can't really explain it properly I'd recommend just rereading those few chapters where they visit the teacher's artist friend
This is the best sequel Oml